The trend today in the field of heavy machinery is toward increasing power and introducing devices which not only make the machinery more efficient but reduce operator fatigue. Some of these labor-saving devices include double-acting hydraulic systems, power steering and power brakes. Power brakes can either be of the drum type or disc type. The disc brake is favored, since it offers more braking capacity than the drum-type brake. The wet type of disc brake is preferred because it can be installed inside the differential housing where it is isolated from the dirt and grime of day-to-day operations.
The wet-type brake is in contact with the lubricating oil. A special oil is required which lubricates all of the relatively moving parts such as the differential gears, transmission gears, etc. At the same time, the lubricating oil must not prevent proper braking action. Furthermore, it must not promote brake chatter. Chatter is caused by slip-stick operation of the brakes when they are in contact with certain types of fluids. Friction-modifying agents, such as dioleylhydrogen phosphite, have been added to tractor hydraulic fluids to reduce chatter. However, lubricants containing such agents have tended to suffer from very high wear rates, particularly at high temperature.
Alkali metal borates are well known in the art and have been used in many lubricants, particularly for their extreme-pressure properties. See, for example, U.S. Pat. Nos. 3,853,772, 3,907,691 and 3,997,454.